N.times.N waveguide grating routers are key components in wavelength division multiplexed (WDM) transmission systems and networks. These devices, also known as wavelength grating multiplexers and frequency routers, are compatible with many lightwave integrated semiconductor applications. These devices are described in U.S. Pat. Nos. 5,002,350, 5,136,671, and 5,243,672, all issued to C. Dragone. They incorporate an N.times.M star coupler as their main building block. This coupler is described in U.S. Pat. No. 4,904,042 issued to Dragone.
In all these router devices, input and output waveguides from the router couple first order diffracted light from the free space region of the router. One object in designing these routers has been to maximize the transfer of first order diffracted optical energy from the waveguide grating region to the input and output waveguides of the router. Another object has been to minimize the amount of energy radiated in higher order diffraction modes from the waveguide grating region in order to minimize the optical loss of the router. Obviously, the higher order diffraction modes are unwanted in such devices.